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Kinetics of flash-induced electron transfer between bacterial reaction centres, mitochondrial ubiquinol: Cytochrome c oxidoreductase and cytochrome c

1983; Elsevier BV; Volume: 725; Issue: 1 Linguagem: Inglês

10.1016/0005-2728(83)90231-1

1879-2650

Qin‐shi Zhu, H.N. van der Wal, Rienk van Grondelle, J.A. Berden,

Spectroscopy and Quantum Chemical Studies

Ascorbate-reduced horse heart cytochrome c reduces photo-oxidized bacterial reaction centres with a second-order rate constant of (5–8) · 108 M−1 · s−1 at an ionic strength of 50 mM. In the absence of cytochrome c, the cytochrome c1 in the ubiquinol:cytochrome c oxidoreductase is oxidized relatively slowly (k = 3.3 · 105 M−1 · s−1). Ferrocytochrome c binds specifically to ascorbate-reduced reductase, with a Kd of 0.6 μM, and only the free cytochrome c molecules are involved in the rapid reduction of photo-oxidized reaction centres. The electron transfer between ferricytochrome c and ferrocytochrome c1 of the reductase is rapid, with a second-order rate constant of 2.1 · 108 M−1 · s−1 at an ionic strength of 50 mM. The rate of electron transfer from the Rieske iron-sulphur cluster to cytochrome c1 is even more rapid. The cytochrome b of the ubiquinol:cytochrome c oxidoreductase can be reduced by electrons from the reaction centres through two pathways: one is sensitive to antimycin and the other to myxothiazol. The amount of cytochrome b reduced in the absence of antimycin is dependent on the redox potential of the system, but in no case tested did it exceed 25% of the amount of photo-oxidized reaction centres.